The photonic lightwave circuit (PLC) serves to integrate various optical components and devices into a functional module for a specific application. They are usually fabricated in a single layer circuit configuration, using a variety of material systems including ion-exchange glass, semiconductors, titanium diffused lithium niobate, polymer on glass and silica on silicon.
Of all the above materials, silica based planar lightwave circuits are attractive as there are many advantages to using silica in planar waveguide devices. The low refractive index step achievable between silica, silicon nitride and silicon allows the production of devices which can be coupled to single-mode silica optical fibre with very high coupling efficiency. Additionally, such devices can be designed to exhibit low sensitivity to temperature and a low polarization dependence. However, this approach necessitates a much larger waveguide core and larger bend radius for curved waveguides, and therefore a much larger overall photonic integrated circuit. Furthermore, active optical devices, such as laser diodes, optical amplifiers and photodetectors, cannot be realized in silica-based layers and therefore hybrid integration of these devices with a silica PLC becomes necessary.
A combination of two or more vertically stacked PLCs serves to accommodate a higher degree of integration of devices with different structural characteristics. The stacking of the PLCs allows for devices/circuits with layers of various thicknesses and refractive indices to be put together in a monolithic arrangement. By way of an example, the design may require the lightwave circuit to possess parts characterized by a small optical mode (suitable for coupling to small mode devices such as semiconductor waveguides and suitable for circuits requiring a small turning radius) and parts characterized by a large optical mode (suitable for coupling to large mode devices such as optical fibre and also suitable for circuits requiring larger waveguides to alleviate sensitivity to fabrication errors). Such a design could not be achieved using conventional PLCs without suffering significant reflection and scattering losses. However, the use of stacked PLCs allows the PLC in one stack to have a small optical mode whilst the PLC in another stack can support a larger optical mode, thereby satisfying the requirements of the example described above.
For stacked PLCs, the amount of optical interference between two vertically adjacent circuits can be controlled over a very wide range by varying the width of the waveguide of the upper layer. In areas where interference is undesirable, the upper waveguide can be widened to provide minimal interference. Conversely, by narrowing the width of the upper waveguide the amount of optical interference can be increased, leading to the possibility of optical cross-connects between an upper and lower PLC at any desired point. Moreover, many properties of the light propagating in such a device can be altered, by controlling the proportion of light in the upper and lower circuits and by introducing differential amounts of birefringence, phase-shift, polarization mode dispersion or attenuation, for example.
Importantly, a combination of layers with high and low refractive index steps provides a valuable interface between single-mode fibre components and III-V compound semiconductor devices, thereby achieving a fully functional module with both active and passive optical components. Consequently, a flexible, cost effective method for creating stacked PLCs of two very different refractive index steps is highly desirable.
Silica based planar waveguide layers are typically fabricated using one of the following deposition technologies: flame hydrolysis (FHD), plasma enhanced chemical vapor deposition (PECVD) and silicon oxynitride or sol-gel deposition. However, when used to fabricate a multilayer PLC, each of these technologies has its drawbacks. FHD entails a long preparation time and expensive drying step, PECVD is slow and produces conformal (not necessarily planar) layers and sol-gel deposition is not suitable for very high refractive index steps leading to higher losses.
The present invention provides a method for fabricating stacked photonic lightwave circuits (PLCs), in which a first PLC is located on a substantially planar cladding layer and a second PLC is stacked vertically above the first PLC and separated from the first PLC by a non-conformal interlayer cladding material that is deposited so as to cover the first PLC and provides a substantially planar surface on which the second PLC is positioned. Subsequently, a further layer of a similar non-conformal interlayer cladding material can be deposited so as to cover the second PLC and act as a passivation layer or to provide a substantially planar surface on which a third PLC can be located. In this way, a stacked PLC device can be fabricated which contains a desired number of vertically spaced coplanar PLCs.
Preferably, the light guiding structures of the PLCs are formed from a material that allows high or low refractive index steps and can be selectively etched for accurate definition of the structures. More preferably, the said structures are formed from a silicon based material such as SiO2 and SiON.
Preferably, the layer of material from which the said structures are fabricated is formed by plasma-enhanced chemical vapour deposition (PECVD).
Preferably, the base cladding layer and subsequent interlayer PLC cladding (IPC) layers are formed from a material whose composition and refractive index can be substantially controlled, so as to permit high and low refractive index steps, and which can be deposited quickly in a thick non-conformal layer to surround the aforementioned structures and provide a substantially planar surface on which to locate the next PLC.
Preferably, the said cladding layers are formed from polymers such as cyclotene resins.
More preferably, the said cladding layers are formed by a sol-gel process.
Preferably, the sol-gel used in the said sol-gel process has been prepared such that the sol phase is sufficiently liquid that it can be deposited, by spin coating or dipping, in a layer of controllable thickness and with a substantially planar surface finish.
Preferably, the said sol-gel is a silica sol-gel.
The use of PECVD deposited dielectric layers for the waveguide structures and sol-gel for the IPC layers provides for etch selectivity, as there is a large difference in etch rate between the two types of layers. This feature facilitates accurate control over the thickness of a particular waveguide in one layer which, if overlapping with a waveguide in a vertically separated layer, allows for interconnecting ports between two PLCs.
Thus the present invention provides a low cost and effective means for fabricating multiply stacked PLCs with planar (non-conformal) cladding layers and with both high and low refractive index steps. Such multiply stacked PLCs have applications in creating devices that require the integration of many complex lightwave functions.